1. Field of the Invention
The present invention relates to a surface-type optical device, such as a vertical cavity surface emitting laser (VCSEL), whose fabrication is easy, whose yield can be increased and which is suitable for use in a two-dimensional array structure, to its fabrication method and to a display device using the surface-type optical device.
2. Related Background Art
Recently, development of a solid-state light emission device of a two-dimensional array type has been desired for the purpose of its applications to large-capacity parallel optical information processing, high-speed optical connection and panel-type display apparatus. Low cost, low consumption of electric power, high productivity, and high reliability are required to achieve those applications. Various materials for such a surface emitting solid-state device have been studied and developed. It has been found that single-crystalline semiconductors are notably suitable for reliability. Especially, development of a surface emitting device using compound semiconductor has been energetically advanced. With this compound semiconductor, light emission is possible over a wide range from ultraviolet to infrared by changing materials of its substrate and layer structure. Hence, that material is prospectively seen as a display device.
Among light emission devices, a laser diode (LD) including reflection mirrors at its opposite ends is most excellent in light emitting efficiency, compared to devices using spontaneous emission. Therefore, electric power consumption can be greatly lowered when those LDs are arranged in a two-dimensional manner. With this point in view, development of the VCSEL has been actively advanced in recent years.
With the VCSEL, devices have also been developed over a range from a blue color at a wavelength of about 400 nm to a communication wavelength band of 1.55 .mu.m. Studies have been made in material series, such as the AlGaN/InGaN series on a sapphire substrate, the InGaAlP/InAlP and InGaAs/AlGaAs series on a GaAs substrate, and the InGaAs/InGaAsP series on an InP substrate.
The fundamental structure of two-dimensional arrayed VCSELs is illustrated in FIG. 1. Laser light is emitted perpendicularly to a substrate 1101. Each device is provided with high-reflective coatings 1109 and 1110 of over 99% reflectivity at opposite end faces of epitaxially-grown layers with a thickness of about several microns. Reference numeral 1114 designates epitaxial layers, reference numeral 1115 designates a light emitting region, and reference numeral 1116 designates an active layer or a light emitting portion.
A multiplicity of alternately-layered layers with different refractive indices and a common .lambda./4 thickness are used as the reflective mirror. The materials are generally dielectric (in the case of FIG. 1), or epitaxially-grown semiconductors. Examples of the epitaxially-grown mirror are as follows: a multi-layer mirror of alternately-formed AlAs and GaAs layers (an AlAs/GaAs mirror), an active layer and other layers are deposited on a GaAs substrate during a single growth process, as disclosed in ELECTRONICS LETTERS, 31, p. 560 (1995); and a GaAs/AlAs mirror formed on a GaAs substrate is directly or without any adhesive bonded to a laser structure of an InGaAsP/InP series grown on an InP substrate, as disclosed in APPLIED PHYSICS LETTERS, 66, p. 1030 (1995).
Further, Japanese Patent Laid-Open No. 9-223848 (1997) discloses a VCSEL with epitaxial semiconductor mirrors, in which after a structure of its epitaxial layers including an active layer grown on a semiconductor substrate is bonded to another semiconductor substrate with an integrated circuit by using polyimide adhesive or the like, its laser substrate is all removed and devices are thus separated from each other, as illustrated in FIG. 2. In such a manner, a semiconductor device with integrated VCSELs and other electric devices is fabricated. In FIG. 2, reference numeral 4100 designates a light input and output substrate, reference numeral 4100A designates a light receiving device, reference numeral 4100B designates a VCSEL, reference numerals 4100C and 4100D designate electric wires of the devices 4100A and 4100B, respectively, reference numeral 2000 designates an integrated-circuit substrate, reference numeral 2000A designates a metal wire of the integrated-circuit substrate 2000, reference numeral 3000 designates an insulating layer, and reference numeral 4000 designates an electric wire. With this construction, a high-density array is possible.
However, where a set of the epitaxial semiconductor layers is used as multi-layer mirror, the difference in refractive indices between the different semiconductor layers cannot be large in the case of InGaAsP/InP, for example. Therefore, the number of the epitaxial layers increases, and its growth time and its thickness are also increased. Thus, its productivity is lowered, and processing of the device and flattening of the surface are difficult to perform.
Further, practical materials for use as the semiconductor mirror are presently GaAs/AlAs, and when their lattice constants are considered, the choice of usable materials of the active layer is limited and its oscillation wavelength band is hence restricted. Where the GaAs/AlAs mirror is directly bonded, the size of the semiconductor substrate is limited although another wavelength band is available due to an expanded range of usable materials for the active layer. This method is thus effective in the case of a small area only.
On the other hand, the dielectric multi-layer mirror cannot be directly laid down on the substrate, although its fabrication is easy. Therefore, the layering needs to be performed after the bottom face of the substrate 1101 is etched to open a window 1101a, as illustrated in FIG. 1. Accordingly, the window 1101a must be precisely formed, and windows 1101a cannot be formed so close to each other. As a result, its yield and uniformity are low or bad, and devices cannot be formed with high density, leading to unsuitability of the devices for a two-dimensional arrayal.
Further, also in the case of the epitaxially-grown mirror, the substrate comes to be an absorptive material depending on the oscillation wavelength band, and thus a hole must be etched in the substrate as illustrated in FIG. 1 when oscillation light is to be taken from the substrate side. Hence, high-density arrayal is also difficult in this case.
Furthermore, in the device as illustrated in FIG. 2, the substrate is not a substrate of transparent material, and therefore light cannot be taken from the substrate side. Further, since an electrode wiring is formed for each light emitting portion at its stepped portion, the wiring process is difficult and its yield is degraded.
Particularly, when the dielectric multi-layer mirror is to be formed on the semiconductor layer of the structure as illustrated in FIG. 2, the yield is drastically injured.